There are variations in the aerotolerance of rumen Prevotella within and between species. Superoxide dismutase (SOD) was detected in some Prevotella spp. Using PCR, the presence of sod genes was shown in all the Prevotella spp. tested and some Bacteroides spp. The PCR DNA fragments allowed partial sequencing of sod from P. ruminicola 118B and P. bryantii TFI-3. Using inverse PCR and by genomic DNA sequencing, the P. ruminicola 118B sod was sequenced. The consequence of disrupting sod on the aerotolerance of B. vulgatus using a ‘suicide’ plasmid was investigated. However, no viable bacteria were isolated. One possible cause of oxygen sensitivity in anaerobes is the cute sensitivity of certain proteins. Work with P. bryantii showed that the cloned enzyme XynB was inactivated by air, whereas its homologue Xsa, from B. ovatus, was not oxygen sensitive. This merited further investigation. Xsa and XynB activity in various gas-saturated buffers was tested. This showed that Xsa needs oxygen to be active. Curiously, Xsa was partially inhibited by CO₂, compared to complete inhibition in nitrogen and argon. This might be explained if traces of oxygen were present in the CO₂-saturated buffer. By contrast, XynB was inactivated by air and nitrogen. For air, it is possible, that nitrogen and not oxygen had an inhibitory effect on XynB via nitric oxide production. Inhibition of XynB by oxygen may have been as a result of inappropriate disulphide bond formation between cysteine residues (Xsa contains two fewer cysteine residues), which takes place during oxidative stress. pRRI2 is a small cryptic plasmid from P. ruminicola 223/M2/7. Sequencing and analysis revealed a 3240-bp plasmid carrying two open reading frames encoding replication and recombination proteins.